Expandable styrene resin beads and foams produced therefrom

ABSTRACT

Foamable modified polystyrene resin particles which are foamable modified polystyrene resin particles (d) comprising modified polystyrene resin particles (c) containing a blowing agent (e); the modified polystyrene resin particles (c) comprising a polystyrene resin and rubber polymer particles (b) being dispersed in the resin, wherein the rubber polymer particles (b) are dispersed uniformly throughout a continuous phase of the polystyrene resin or in a state of a density of the particles (b) being higher in a center portion of the resin particle than in a surface layer portion thereof, and the modified polystyrene resin particles (c) are modified polystyrene resin particles prepared by impregnating a monomer mixture consisting essentially of a styrene monomer and a conjugated diene monomer and comprising the styrene monomer in a larger amount than the conjugated diene monomer, into polystyrene resin particles (a) and carrying out copolymerization.

TECHNICAL FIELD

The present invention relates to foamable modified polystyrene resinparticles, a process for preparation of the resin particles, a foamedarticle which is produced by using the resin particles and has excellentbalance of break resistance and cushioning property and a process forproduction of the foamed article.

BACKGROUND ART

It is well known that a foamed article produced by expanding foamablepolystyrene resin particles obtained by impregnating polystyrene resinparticles with a blowing agent is broken easily. To improve such adefect, there has been proposed a process for mechanically blending astyrene-butadiene block copolymer with a polystyrene resin (cf.JP-B-47-17465 and JP-A-54-158467), a process for mixing astyrene-butadiene block copolymer and a polybutadiene resin in a solventto obtain a phase-separated microstructure by a solvent casting method(cf. JP-A-56-67344) and a process employing particles of a resin,so-called high impact polystyrene resin (HIPS) particles which areexcellent in impact strength and are prepared by polymerizing a styrenemonomer in the presence of conjugated diene polymer (JP-B-47-18428 andJP-A-7-90105).

However it is necessary that those foamable polystyrene resin particlesare subjected to pelletizing through extrusion molding, which results inincrease in cost. Further in case of a commercially available highimpact polystyrene resin, a molecular weight of its polystyrene cannotbe increased beyond a certain level.

As a method for improving break resistance at low cost, an impregnationpolymerization method for polymerizing by impregnating styrene polymerparticles with a conjugated diene monomer, or the like.

For example, JP-A-6-49263 discloses foamable polystyrene resin particlesprepared by copolymerizing a monomer mixture of a styrene monomer and aconjugated diene monomer in a state of the monomer mixture beingimpregnated around the surface portion of polystyrene resin particles togive polystyrene resin particles containing the obtained conjugateddiene copolymer gathering densely around the surface portion of theresin particle, and then impregnating the polystyrene resin particleswith a blowing agent. Also it is disclosed that the center portion ofthe foamable resin particle is rich with polystyrene and, since theblowing agent is kept in the center portion, the resin particles havinghigh expansion ratio can be obtained. Further it is disclosed that themonomer mixture is a mixture of 10 to 55% by weight of styrene monomerand 90 to 45% by weight of conjugated diene monomer. However in all theexamples thereof, the conjugated diene monomer is used in an amount ofnot less than 1.5 times that of the styrene monomer and an object ofthat invention is to obtain polystyrene resin particles, in which aconjugated diene copolymer gathers densely around the surface portion ofthe resin particle and the center portion of the foamable resin particleis rich with polystyrene. From that point of view, only a monomermixture comprising the conjugated diene monomer in a larger proportionessentially is disclosed.

The process mentioned above does not require a pelletizing step and isexcellent from the viewpoint of cost. However the obtained foamedarticle, particularly the foamed article produced by moldingpre-expanded particles within a mold is low in percentage of fusedparticles (fusion rate) and is insufficient in break resistance (inimpact strength by a falling weight method) because the conjugated dienepolymer particles are present densely around the surface portion of theresin particle.

Also in WO98/29485, there is disclosed foamable modified polystyreneresin particles obtained by polymerizing a conjugated diene monomer inpolystyrene resin particles to form rubber particles of conjugated dienepolymer in a uniformly dispersed state in the polystyrene resinparticles, impregnating and polymerizing a styrene monomer in theobtained polystyrene resin containing rubber particles and thenimpregnating the obtained modified polystyrene resin with a blowingagent. Those foamable resin particles are characterized in that whenthey are expanded, there is substantially no deformation of the rubberparticles before and after the expansion and an Izod impact strength ofthe modified polystyrene resin itself before expanded is lower than thatof conventional resin called high impact polystyrene (HIPS). Also thoughthe publication WO98/29485 discloses impregnation and polymerization ofa styrene monomer and a conjugated diene monomer, a mixing ratio thereofdisclosed is only 5:10 (weight ratio) in example of the publication. Inthis case, too, a monomer mixture being rich with the conjugated dienemonomer is used.

The process mentioned above does not necessitate a pelletizing step andis excellent from the viewpoint of cost. Also a fusion rate ofpre-expanded particles in a foamed article obtained by expanding them inmold is high and a break resistance (impact strength by a falling weightmethod) is improved as compared with the foamed article disclosed in theabove-mentioned JP-A-6-49263 but an obtained foamed article is not equalto a foamed article of commercially available high impact polystyrene(HIPS) in case of expanding at high expansion ratio (about 50 times ormore) into a foamed article having a density of not more than 0.02g/cm³.

As mentioned above, the impregnation polymerization method does notnecessitate a pelletizing step and can enhance break resistance ofpolystyrene resin foamed article at low cost. However it was difficultto attain break resistance equal to that of a foamed article obtained byusing high impact polystyrene (HIPS) particles.

An object of the present invention is to provide foamable modifiedpolystyrene resin particles which can be produced without a pelletizingstep and gives a foamed article having excellent break resistance andcushioning property even at high expansion ratio, a process forpreparation thereof and a foamed article obtained by expanding the resinparticles.

DISCLOSURE OF INVENTION

Namely the present invention relates to foamable modified polystyreneresin particles which are foamable modified polystyrene resin particles(d) comprising modified polystyrene resin particles (c) containing ablowing agent (e); the modified polystyrene resin particles (c) compriserubber polymer particles (b) dispersed in a polystyrene resin,

wherein the rubber polymer particles (b) are dispersed uniformlythroughout a continuous phase of the polystyrene resin or in a state ofa density of the particles (b) being higher in a center portion of theresin particle than in a surface layer portion thereof, and

the modified polystyrene resin particles (c) are modified polystyreneresin particles (c1) obtained by impregnating a monomer mixtureconsisting essentially of a styrene monomer and a conjugated dienemonomer into polystyrene resin particles (a) and carrying outcopolymerization, said monomer mixture being containing the styrenemonomer in a larger amount than the conjugated diene monomer.

The modified polystyrene resin particles (c) may be modified polystyreneresin particles (c2) obtained by impregnating the monomer mixtureconsisting essentially of the styrene monomer and the conjugated dienemonomer in the polystyrene resin particle (a) and carrying ourcopolymerization; said monomer mixture containing the styrene monomer ina larger amount than the conjugated diene monomer to give the modifiedpolystyrene resin particles (c1) and further subjecting the obtainedpolystyrene resin particles to impregnation polymerization of a styrenemonomer.

It is preferable that the above-mentioned rubber polymer particles (b)consists essentially of a copolymer of the styrene monomer and theconjugated diene monomer.

It is preferable that the modified polystyrene resin particles (c1) areprepared by impregnating 40 to 10 parts by weight of the monomer mixtureinto 60 to 90 parts by weight of polystyrene polymer particle (a) andcarrying out copolymerization; said monomer mixture comprising thestyrene monomer of more than 55% by weight and not more than 90% byweight and the conjugated diene monomer of more than 10% by weight andless than 45% by weight.

Provided that when the foamable modified polystyrene resin particles (d)are molded into a modified polystyrene resin foamed article having adensity of 0.02 g/cm³, a cushioning coefficient of the foamed article isrepresented by A and a 50% failure height thereof according to fallingweight method is represented by B, and that when foamable un-modifiedpolystyrene resin particles (f) prepared by impregnating the polystyreneresin particles (a) with the blowing agent (e) are molded into anun-modified polystyrene resin foamed article having a density of 0.02g/cm³, a cushioning coefficient of the foamed article is represented byC and a 50% failure height thereof according to falling weight method isrepresented by D, it is preferable that the modified polystyrene resinfoamed article has a relation that A/C is 1.00 to 1.08 and B/D is 1.35to 2.35.

Also it is preferable that the rubber polymer particles (b) beingpresent in the center portion of the foamable modified polystyrene resinparticles (d) have a circle equivalent diameter of average area of 0.01to 0.20 μm or a gel content of the modified polystyrene resin particles(c) is 15 to 40% by weight.

Further it is preferable that the above-mentioned rubber polymerparticles (b) are flattened in cell membrane of the modified polystyreneresin foamed article obtained by expanding the foamable modifiedpolystyrene resin particles, and particularly an average flatness of therubber polymer particles (b) flattened in a cell membrane of themodified polystyrene resin foamed article obtained by expanding thefoamable modified polystyrene resin particles (d) so that the foamedarticle has a density of 0.02 g/cm₃ is within a range of 1.1 to 9.

Further the present invention relates to foamable modified polystyreneresin particles which are foamable modified polystyrene resin particles(d) comprising modified polystyrene resin particles (c) containing ablowing agent (e); the modified polystyrene resin particles (c) compriserubber polymer particles (b) dispersed in a polystyrene resin,

wherein the rubber polymer particles (b) are dispersed uniformlythroughout a continuous phase of the polystyrene resin or in a state ofa density of the particles (b) being higher in a center portion of theresin particle than in a surface layer portion thereof, and

provided that when the foamable modified polystyrene resin particles (d)are molded into a modified polystyrene resin foamed article having adensity of 0.02 g/cm³, a cushioning coefficient of the foamed article isrepresented by A and a 50% failure height thereof according to fallingweight method is represented by B, and that when foamable unmodifiedpolystyrene resin particles (f) prepared by impregnating polystyreneresin particles (a) with a blowing agent (e) are molded into anun-modified polystyrene resin foamed article having a density of 0.02g/cm³, a cushioning coefficient of the foamed article is represented byC and a 50% failure height thereof according to falling weight method isrepresented by D, the modified polystyrene resin foamed article has arelation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.

Also in those foamable modified polystyrene resin particles, it isdesirable that the above-mentioned rubber polymer particles (b) areparticles of a copolymer consisting essentially of the styrene monomerand the conjugated diene monomer.

Also it is preferable that the rubber polymer particles (b) beingpresent in the center portion of the foamable modified polystyrene resinparticles (d) have a circle equivalent diameter of average area of 0.01to 0.20 μm or a gel content of the modified polystyrene resin particles(c) is 15 to 40% by weight.

Still further the present invention relates to foamable modifiedpolystyrene resin particles which are foamable modified polystyreneresin particles (d) comprising modified polystyrene resin particles (c)containing a blowing agent (e); the modified polystyrene resin particles(c) comprise rubber polymer particles (b) dispersed in a polystyreneresin,

wherein the rubber polymer particles (b) are dispersed uniformlythroughout a continuous phase of the polystyrene resin or in a state ofa density of the particles (b) being higher in a center portion of theresin particle than in a surface layer portion thereof, and

the rubber polymer particles (b) are flattened in a cell membrane of amodified polystyrene resin foamed article obtained by expanding thefoamable modified polystyrene resin particles (d).

It is desirable that the above-mentioned rubber polymer particles (b)are particles of a copolymer consisting essentially of the styrenemonomer and the conjugated diene monomer.

Provided that when the foamable modified polystyrene resin particles (d)are molded into a modified polystyrene resin foamed article having adensity of 0.02 g/cm³, a cushioning coefficient of the foamed article isrepresented by A and a 50% failure height thereof according to fallingweight method is represented by B, and that when foamable un-modifiedpolystyrene, resin particles (f) prepared by impregnating polystyreneresin particles (a) with a blowing agent (e) are molded into anun-modified polystyrene resin foamed article having a density of 0.02g/cm³, a cushioning coefficient of the foamed article is represented byC and a 50% failure height thereof according to falling weight method isrepresented by D, it is preferable that the modified polystyrene resinfoamed article has a relation that A/C is 1.00 to 1.08 and B/D is 1.35to 2.35.

Also it is preferable that the rubber polymer particles (b) beingpresent in the center portion of the foamable modified polystyrene resinparticles (d) have a circle equivalent diameter of average area of 0.01to 0.20 μm or a gel content of the modified polystyrene resin particles(c) is 15 to 40% by weight.

It is preferable that an average flatness of the rubber polymerparticles (b) flattened in a cell membrane of the modified polystyreneresin foamed article obtained by expanding the foamable modifiedpolystyrene resin particles (d) and having a density of 0.02 g/cm³ is ina range of 1.1 to 9.

Also the present invention relates to the modified polystyrene resinfoamed article having a cell membrane of modified polystyrene resincomprising a polyester resin and rubber polymer particles (b) dispersedin the resin,

wherein the rubber polymer particles (b) are flattened in the cellmembrane, and an average flatness of the flattened rubber polymerparticles (b) in the cell membrane of the modified polystyrene resinfoamed article having a density of 0.02 g/cm³ is in a range of 1.5 to 8.

It is desirable that the above-mentioned rubber polymer particles (b)are particles of a copolymer substantially comprising the styrenemonomer and the conjugated diene monomer.

Provided that a cushioning coefficient of the modified polystyrene resinfoamed article having a density of 0.02 g/cm³ is represented by A and a50% failure height thereof according to falling weight method isrepresented by B, and that a cushioning coefficient of a foamed articleof unmodified polystyrene resin containing no rubber polymer particlesand having a density of 0.02 g/cm³ is represented by C and a 50% failureheight thereof according to falling weight method is represented by D,it is preferable that the modified polystyrene resin foamed article hasa relation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.

Further the present invention relates to the modified polystyrene resinfoamed article obtained by expanding the above-mentioned foamablemodified polystyrene resin particles and the process for producing themodified polystyrene resin foamed article by expanding theabove-mentioned foamable modified polystyrene resin particles.

Still further the present invention relates to the process for producingthe foamable modified polystyrene resin particles which comprises:

impregnating a monomer mixture consisting essentially of the styrenemonomer and the conjugated diene monomer and comprising the styrenemonomer in a larger amount than the conjugated diene monomer, into thepolystyrene resin particle (a) and carrying out copolymerization to givemodified polystyrene resin particles (c1),

wherein rubber polymer particles (b) are dispersed uniformly throughouta continuous phase of the polystyrene resin or in a state of a densityof the particles (b) being higher in a center portion of the resinparticle than in a surface layer portion thereof, and then

impregnating the modified polystyrene resin particles (c1) with ablowing agent (e),

and further relates to the process for producing the foamable modifiedpolystyrene resin particles which comprises:

impregnation-polymerizing the modified polystyrene resin particles (c1)with a styrene monomer to give the modified polystyrene resin particles(c2) and then

impregnating the modified polystyrene resin particles (c2) with theblowing agent (e).

In the impregnation polymerization of those production processes forobtaining the modified polystyrene resin particles (c1), it ispreferable that 40 to 10 parts by weight of the monomer mixture isimpregnated into 60 to 90 parts by weight of the polystyrene polymerparticles (a) and the monomer mixture comprising a styrene monomer ofmore than 55% by weight and not more than 90% by weight and a conjugateddiene monomer of more than 10% by weight and less than 45% by weight isused.

The “foamed article” used herein encompasses pre-expanded particlesobtained by expanding the foamable resin particles and a molded articleobtained by molding the pre-expanded particles in a mold, both of whichare described hereinafter. Unless otherwise noted, the word “foamedarticle” means the both of them.

Prior to explaining the present invention, explanations of the wordsused in the present invention are made below.

In the present invention, the state of the rubber polymer particles (b)“being dispersed uniformly throughout the polystyrene resin particle (a)or a density thereof being higher in a center portion of the resinparticle than in a surface layer portion of the resin particle” meansthat the distributing state of the rubber polymer particles (b) does notdiffer between the surface layer portion and the center portion of thepolystyrene resin particle (a) or is sparser in the surface layerportion.

In the present invention, the “surface layer portion” means a portiondefined between the depth of 2 μm to 6 μm from the surface of themodified polystyrene resin particle (c) toward the center thereof. The“center portion” means a sphere portion having a radius of 50 μm, thecenter of said sphere portion being the center of the modifiedpolystyrene resin particle, provided that the particle is a sphere.

The concrete measuring method is explained below by using photographs ofthe modified polystyrene resin particle prepared in Example 2 mentionedhereinafter, which were taken with a transmission electron microscope(TEM). FIG. 1 is a TEM photograph (×40000) of the “center portion”, andFIG. 2 is a TEM photograph (×40000) of the “surface layer portion”.FIGS. 3 (In FIG. 3, X mark represents the center of the particle) and 4are photocopies of FIGS. 1 and 2, respectively for measuring an areapercentage (%) of the rubber particles.

The area percentage (%) of the rubber particles is measured andcalculated as follows.

First an area A (1.25 μm×1.25 μm) surrounded by a full line in FIGS. 3and 4 is enlarged by four times with a copying machine, respectively,and a weight thereof is measured. Black region (rubber polymer particlesdyed with osmium oxide) are cut off (in case of rubber polymer particlescontaining occlusion polystyrene particles therein, a region of therubber polymer particles containing the occlusion polystyrene particlesis cut off as it is), and a weight of the cut portions B (or remainingportions after cutting off) is measured. The area percentage (%) of therubber particles is represented by (cut portions B/total area A)×100. InFIG. 1 (FIG. 3) and FIG. 2 (FIG. 4), the area percentage is 38.4% and25.5%, respectively.

Then the “area ratio of rubber particles” of the “surface layer portion”to the “center portion” is calculated. $\begin{matrix}{{Area}\quad {ratio}\quad {of}} \\{{rubber}\quad {particles}}\end{matrix} = \frac{\begin{matrix}{{Area}\quad {percentage}\quad (\%)\quad {of}\quad {rubber}} \\{{particles}\quad {of}\quad {surface}\quad {layer}\quad {portion}}\end{matrix}}{\begin{matrix}{{Area}\quad {percentage}\quad (\%)\quad {of}\quad {rubber}} \\{{particles}\quad {of}\quad {center}\quad {portion}}\end{matrix}}$

In the present invention, the state of “being uniformly dispersedthroughout or a density being higher in the center portion than in thesurface layer portion” is as mentioned above. From an aspect of “an arearatio of the rubber particles”, the area ratio is preferably from 0.10to 1.15, more preferably 0.30 to 1.15, particularly preferably 0.50 to1.15.

When the area ratio of rubber particles exceeds 1.15, there is atendency that fusion between the pre-expanded particles at the time ofin-mold molding is inferior. Since the foamable modified polystyreneresin particles of the present invention are uniform throughout or adensity of the rubber polymer particles is higher in the center portionthan in the surface layer portion, the fusion between the pre-expandedparticles is easy and in-mold molding is easy.

In the present invention, the “cushioning coefficient ratio (A/C)” meansthe following value.

First the cushioning coefficient is a minimum cushioning coefficientwhen a 14 kg of a weight is dropped from a height of 60 cm onto a 90mm×90 mm×50 mm sample of expanded foamed article and the sample isdeformed by 50 to 65% as compared with the sample before the testaccording to JIS Z 0235. In the concrete measurement and calculation,first a reference cushioning coefficient C of a foamed article obtainedby in-mold molding of un-modified polystyrene resin particles under theexpansion-molding conditions mentioned hereinafter is measured, and thena cushioning coefficient A of a foamed article obtained byexpansion-molding of polystyrene resin particles to be tested under thesame expansion-molding conditions is measured. The A/C is called acushioning coefficient ratio. The smaller the ratio is, the better thecushioning characteristic is.

The “impact strength ratio (B/D) according to the falling weight method”of the present invention means a value obtained as follows.

According to JIS K 7211, a 321 g steel ball is dropped on a 200 mm×20mm×40 mm test piece of a foamed article and a 50% failure height whichcauses rupture of 50% of the number of test pieces of the foamed articleis calculated by the following equation:$H_{50} = {H_{i} + {d\left\lbrack {\frac{\sum\left( {i \cdot n_{i}} \right)}{N} \pm 0.5} \right\rbrack}}$

where H₅₀: A 50% failure height (cm) which causes rupture of 50% of thenumber of test pieces of the foamed article

H_(i): Test height (cm) when the height level (i) is zero, representingthe height anticipated to cause the rupture of the test piece

d: Height interval (cm) for increasing or decreasing the test height

i: Height level which increases or decreases by one, taking the value ofzero when the test height is H₁ (i= - - - −3, −2, −1, 0, 1, 2, 3, - - -)

n_(i): Number of test pieces which break (or do not break) at eachheight level

N: Total number of test pieces which break (or do not break) (N=Σn_(i))

Whichever is the larger of the number of broken and unbroken test piecesshall be used. In case where the numbers are the same, either one may beused.

±0.5: This value shall be negative when using the data when broken andbe positive when using the data when unbroken.

In the concrete measurement and calculation, first a reference 50%failure height D of a foamed article obtained by in-mold molding ofun-modified polystyrene resin particles under the expansion-moldingconditions mentioned hereinafter is measured, and then a 50% failureheight B of a foamed article obtained by expansion-molding ofpolystyrene resin particles to be tested under the sameexpansion-molding conditions is measured. The B/D is called an impactstrength ratio according to the falling weight method. The larger theratio is, the higher the break resistance is.

In the present invention, the “density” means the following value.

A density D (g/cm³) of the foamed article is obtained by the followingequation according to JIS K 6767.

D=G/V

wherein

G: Weight of foamed article (g),

V: Volume of foamed article (cm₃)

V is calculated by cutting a part of the foamed article into arectangular parallelepiped and measuring a depth, width and heightthereof. Measuring tools and accuracy of measurement are according toJIS K 6767.

The expansion-molding conditions for the measurements of theabove-mentioned impact strength ratio and cushioning coefficient ratioare as follows. Those conditions are used also in examples andcomparative examples described hereinafter.

Pre-Expansion Conditions

Foamable resin particles

Average particle size: 1 mm

Blowing agent: Pentane (normal/iso=40/60)

A normal pressure pre-expanding equipment which was pre-heated ischarged with 1 to 2 kg of foamable resin particles, followed by passingsteam at about 0.061 MPa and introducing air properly with stirring.Thus the particles are expanded up to a given expansion ratio in about 1minute to about 3 minutes.

In-Mold Molding Conditions

After molding under the following conditions, a foamed article isallowed to stand for vacuum cooling.

Molding machine: TH90VMII available from Toyo Kikai Kinzoku KabushikiKaisha

Density after molding: 0.02 g/cm³ (expansion ratio: about 50 times)

Molding conditions:

Pre-steaming: 3 seconds

One-way steaming: 4 seconds

Opposite steaming: 1 second

Autoclave steaming: 12 seconds

Additional steaming: 3 seconds

Retaining: 3 seconds

Water cooling: 20 seconds

Set steam pressure at the heating from both sides:

Cavity/core=0.066/0.087 (MPa)

In the present invention the “average flatness” of the rubber polymerparticles (b) in the cell membrane of the foamed article is determinedas follows. Those conditions are used also in examples and comparativeexamples described hereinafter.

The foamable resin particles are expansion-molded by the above-mentionedmethod to give a foamed article having a density of 0.02 g/cm³. Therubber polymer particles in the cell membrane of the foamed article aredyed with osmium oxide, followed by viewing with a transmission electronmicroscope (JEM-1200EX available from Nippon Denshi Kabushiki Kaisha,×7200 to ×40000). Then with respect to 100 flattened rubber polymerparticles, a dimension thereof in the cell membrane direction and adimension in a thickness direction thereof are measured. A valueobtained by dividing the dimension in the cell membrane direction by thedimension in the thickness direction is assumed to be a flatness. Anaverage value of the flatness of the rubber polymer particles having aflatness of not less than 1.1 is assumed to be an average flatness.

In the present invention the “circle equivalent diameter of averagearea” of the rubber polymer particles (b) in the center portion of thefoamable modified polystyrene resin particle (d) is determined asfollows. Those conditions are used also in examples and comparativeexamples described hereinafter.

The rubber polymer particles in the foamable resin particle are dyedwith osmium oxide, and with a transmission electron microscope(JEM-1200EX available from Nippon Denshi Kabushiki Kaisha, ×40000), aphotograph was taken. An area of 3.76 μm² calculated from a scale (0.5μm) on the photograph was enlarged by four times and photocopied and ablack rubber portion dyed with osmium oxide of the photocopy is cut off(in case of rubber polymer particles containing occlusion polystyreneparticles therein, a region of the rubber polymer particles containingthe occlusion polystyrene particles is cut off as it is), and a weight Eg of all the cut portions of rubber polymer particles is measured. Thenthe weight E g of the cut pieces of paper is divided by the number F ofrubber polymer particles and an average weight G g of the rubber polymerparticles is calculated. Separately a portion of the photocopy includingthe scale (0.5 μm) was enlarged by four times similarly and photocopied.The enlarged portion equivalent to 0.25 μm² is cut off and its weight ismeasured to determine a weight H g of the paper equivalent to 0.25 μm².An area per 1 g of paper, i.e. I μm²/g is calculated by dividing 0.25μm² by the paper weight H g equivalent to 0.25 μm². Then an average areaJ μm² of the rubber polymer particles is calculated by multiplying theaverage weight G g of the rubber polymer particles by the area per 1 gof paper, i.e. I μm²/g. This average area J μm² of the rubber polymerparticles is divided by number π and a root of the obtained value ismultiplied by 2. The thus obtained value is assumed to be a circleequivalent diameter of average area K μm. Herein the measurements weremade by using FIGS. 1 and 2 used for obtaining the rubber portion arearatio.

The “center portion” means a sphere portion having a radius of 50 μm,the center of said sphere portion being the center of the foamablemodified polystyrene resin particle, provided that the particle is asphere.

In the present invention the “gel content” is obtained as follows.

0.5 Gram of foamable modified polystyrene resin particles is put in asolution comprising 31.8 ml of methyl ethyl ketone and 3.2 ml ofmethanol. After stirring for eight hours or more, a portion of solution(dissolved portion) is separated from a gel portion (insoluble portion)by centrifugal separation (for 30 minutes at 15,000 rpm), and theportion of solution is removed. Then a solution comprising 31.8 ml ofmethyl ethyl ketone and 3.2 ml of methanol is added, followed bystirring at 30° C. for 30 minutes and separating a portion of solutionfrom a gel portion by centrifugal separation (for 30 minutes at 15,000rpm). The portion of solution is removed. Further a solution comprising31.8 ml of methyl ethyl ketone and 3.2 ml of methanol is added, followedby stirring at 50° C. for 30 minutes and separating a portion ofsolution from a gel portion by centrifugal separation (for 30 minutes at15,000 rpm). Then the portion of solution is removed and the obtainedgel portion is dried at 60° C. for eight hours to measure a weight ofthe gel portion. Thus a weight of the gel portion contained in 0.5 g offoamable modified polystyrene resin particles is calculated. Theobtained value is assumed to be a gel content.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph (×40000) of a transmission electron microscopeshowing the center portion of the foamable modified polystyrene resinparticle obtained in Example 2 of the present invention.

FIG. 2 is a photograph (×40000) of a transmission electron microscopeshowing the surface portion of the foamable modified polystyrene resinparticle obtained in Example 2 of the present invention.

FIG. 3 is a photocopy of FIG. 1 for explaining measuring methods of anarea percentage of rubber particles and an area ratio of rubberparticles.

FIG. 4 is a photocopy of FIG. 2 for explaining measuring methods of anarea percentage of rubber particles and an area ratio of rubberparticles.

FIG. 5 is a photograph (×40000) of a transmission electron microscopeshowing the modified polystyrene resin foamed article obtained inExample 2 of the present invention.

FIG. 6 is a photograph (×40000) of a transmission electron microscopeshowing the foamed article obtained in Comparative Example 3.

FIG. 7 is a photograph (×40000) of a transmission electron microscopeshowing the foamed article obtained in Comparative Example 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Firstly a preferred preparation process of the foamable modifiedpolystyrene resin particles of the present invention is explained below.

This preparation process is a process for producing the modifiedpolystyrene resin particles (c) by impregnating the monomer mixtureconsisting essentially of the styrene monomer and the conjugated dienemonomer into the polystyrene polymer particles (a) and then carrying outcopolymerization with the monomer mixture being containing the styrenemonomer in a larger amount than the conjugated diene monomer. Thisimpregnation polymerization is hereinafter referred to as the firstimpregnation polymerization, and the obtained polystyrene resinparticles are called the modified polystyrene resin particles (c1). Ifnecessary, the resin particles may be subjected to the secondimpregnation polymerization described later.

The preparation process is analogous to the preparation processesdescribed in the above-mentioned JP-A-6-49263 and WO98/29485, butsubstantially differs from them from a point that in the monomer mixtureused in the impregnation polymerization, a proportion of the styrenemonomer is larger than that of the conjugated diene monomer. As aresult, there arise a big difference in physical properties andperformance with respect to the obtained modified resin particles andfoamed article obtained by expanding them.

In cases of the inventions disclosed in the above-mentionedpublications, a rubber component is intended to be increased, and as aresult, a mixing ratio of the conjugated diene monomer is increased.

However in the present invention, a conception is entirely reversed, andan amount of the styrene monomer is made larger than that of theconjugated diene monomer in the monomer mixture.

As a result,

(1) since the rubber polymer particles (b) are dispersed uniformlythroughout a continuous phase of the polystyrene resin or in a state ofa density of the particles (b) being higher in the center portion of theresin particle than in the surface layer portion thereof, good fusionbetween the pre-expanded particles is obtained when a foamed article isproduced by in-mold molding, and further

(2) in the modified polystyrene resin foamed article, the rubber polymerparticles (b) dispersed in the cell membrane of the obtained modifiedpolystyrene resin are properly flattened and thereby a foamed articlehaving an excellent break resistance and a good balance of breakresistance and cushioning property can be provided.

The foamable modified polystyrene resin particles (d) of the presentinvention explicitly differs from the resin particles of JP-A-6-49263 inthe above-mentioned point (1) because rubber polymer particles of thelatter resin particles are present densely in a surface portion. Furtherfrom the viewpoint of the above (2), the resin particles of the presentinvention explicitly differs from resin particles of WO98/29485 in whichthere is substantially no deformation of the rubber particles before andafter the expansion. Further from the viewpoint of the above (2), theresin particles of the present invention differs from conventional highimpact polystyrene resin (HIPS) particles in which rubber polymerparticles are flattened in the form of string in a cell membrane whenexpanded. The difference from HIPS is described later.

Explained below are preparation steps and conditions. However thepresent invention is not limited to them.

First preferably 40 to 10 parts by weight of monomer mixture comprisingthe styrene monomer and the conjugated diene monomer and comprising thestyrene monomer in a larger amount than the conjugated dine monomer isimpregnated into preferably 60 to 90 parts by weight of polystyreneresin particles (a) and polymerization is carried out. Thispolymerization is called the first impregnation polymerization. Themonomers to be mixed may be mixed in the polystyrene resin particles.The styrene monomer and conjugated diene monomer may be impregnatedseparately in the polystyrene resin particles. When the polystyreneresin particles (a) are less than 60 parts by weight, in some cases apolymerization system becomes unstable when impregnating the monomermixture. When more than 90 parts by weight, there is a case where breakresistance of a foamed article produced from the obtained resinparticles becomes insufficient. An amount of the polystyrene resinparticles (a) is further preferably 65 to 85 parts by weight.

In the monomer mixture, an amount of the styrene monomer is larger thanthat of the conjugated diene monomer, and is preferably more than 55% byweight and not more than 90% by weight, more preferably 60to 85% byweight, further preferably 60to 80% by weight from the viewpoint ofenhancing break resistance.

In the impregnation of the monomer mixture into the polystyrene resinparticles (a), it is important to carry out the impregnation of themonomer mixture sufficiently into the inside of the polystyrene resinparticles (a) before the polymerization.

With respect to the polystyrene resin particles (a) which are rawmaterials to be used in the present invention, there may be usedconventional polystyrene resin. Examples thereof are styrene homopolymerand a copolymer of styrene with other monomer. Examples of the othermonomer are an unsaturated fatty acid ester such as methyl methacrylateor butyl acrylate, an unsaturated fatty acid such as acrylic acid,methacrylic acid or maleic anhydride, α-methylstyrene, andacrylonitrile. Unlike the conventional high impact polystyrene (HIPS), aweight average molecular weight can be adjusted optionally in a range ofabout 150,000 to about 600,000. The weight average molecular weight isnot less than about 250,000, preferably not less than 300,000 from theviewpoint of giving good break resistance to a foamed article, and isnot more than about 500,000, preferably not more than about 400,000 froma point that an expansion rate is within a proper range.

As the polystyrene resin particles (a), since those prepared throughconventional suspension polymerization method can be used as they are,no specific pelletizing step is required. It is a matter of course thatthose obtained by bulk polymerization, emulsion polymerization, solutionpolymerization or the like, and pulverized if necessary can be used. Itis preferable that the average particle size is from about 0.1 mm toabout 3 mm, usually from 0.5 to 1.5 mm.

To the raw polystyrene resin particles (a) may be added conventionaladditives such as a nucleating agent, filler, plasticizer, flameretarder, lubricant, coloring agent, ultraviolet ray absorber andantioxidant in an amount not impairing effects of the present invention.

Then the monomer mixture to be used in the present invention isexplained below. Examples of the conjugated diene monomer to be used inthe present invention are one or more of butadiene, isoprene,chloroprene, 1,4-pentadiene, 1,5-hexadiene and the like. Butadiene isparticularly preferable. An amount the conjugated diene monomer to besubjected to the impregnation polymerization is preferably from 3 to 18%by weight, more preferably from 5 to 15% by weight, particularlypreferably from 5 to 10% by weight on the basis of the modifiedpolystyrene resin particles (a) from a point of giving a good breakresistance to a foamed article.

As the styrene monomer, there are styrene derivatives in addition tostyrene. Examples of the styrene derivative are, for instance,α-methylstyrene, α-ethylstyrene, α-chlorostyrene and divinylbenzene.Those styrene monomers can be used solely or in a mixture thereof.Divinylbenzene is useful in case of a combination use with a styrenemonomer other than divinylbenzene.

Further non-essential other monomer may be added as a copolymerizingcomponent to an extent not changing properties of the rubber polymerparticles (b) substantially.

The polymerization of the monomer mixture is carried out by using apolymerization initiator. The polymerization may be carried out byimpregnating the monomer mixture to which the initiator is previouslyadded, into the resin particles (a). It is preferable, in order toobtain the rubber polymer particles (b) in more uniform dispersionstate, that the polymerization is carried out after impregnating themonomer mixture uniformly into the resin particles and then impregnatingthe initiator uniformly.

Examples of the polymerization initiator are, for instance, radicalpolymerization initiators such as benzoyl peroxide, lauroyl peroxide,t-butyl peroxybenzoate and1,1-di-t-butylperoxy-2,4-di-t-butylcyclohexane. The amount of thepolymerization initiator is from 0.05 to 5% by weight, preferably 0.1 to2% by weight on the basis of the conjugated diene monomer.

The first impregnation polymerization can be carried out in the systemwhere the polystyrene resin particles (a) are dispersed and suspended inwater by impregnating with the monomer mixture and polymerizationinitiator uniformly at a polymerization temperature (usually 50° to 100°C.) or a temperature lower than that and then holding at thepolymerization temperature for 1 to 20 hours (impregnation suspensionpolymerization).

In case of the suspension impregnation polymerization, in order toenhance dispersibility of the polystyrene resin particles (a), there canbe used a dispersing agent, for example, an organic dispersing agentsuch as partly saponified polyvinyl alcohol, polyacrylate, polyvinylpyrrolidone, carboxymethyl cellulose or methyl cellulose; an inorganicdispersing agent such as calcium pyrophosphate, calcium phosphate,calcium carbonate, magnesium pyrophosphate, magnesium phosphate,magnesium carbonate or magnesium oxide, and the like. In case where theinorganic dispersing agent is used, a combination use of a surfactant isdesirable.

In the obtained modified polystyrene resin particles (c1), the rubberpolymer particles (b) are dispersed uniformly throughout a continuousphase of the polystyrene resin or in a state of a density of theparticles (b) being higher in the center portion of the resin particlethan in the surface layer portion thereof. It is preferable that acircle equivalent diameter of average area of the rubber polymerparticles (b) being present in the center portion is 0.01 to 0.20 μm.

From the viewpoint of enhancing fusing property at the time whenpre-expanded particles are fused by in-mold molding, the modifiedpolystyrene resin particles (c1) may be further impregnation-polymerizedwith a styrene monomer. This impregnation polymerization is calledsecond impregnation polymerization. The modified polystyrene resinparticles obtained by the second impregnation polymerization are calledmodified polystyrene resin particles (c2). Examples of the styrenemonomer used in the second impregnation polymerization are the styrenemonomers exemplified in the first impregnation polymerization. Thestyrene monomer used may be the same as or different from that in themonomer mixture.

The amount of the styrene monomer used in the second impregnationpolymerization is from 3 to 50% by weight, preferably from 5 to 30% byweight, further preferably 5 to 20% by weight on the basis of thepolystyrene resin particles (a). When more than 50% by weight, there isa case where the weight amount of the rubber polymer particles becomesrelatively too small and break resistance of a foamed article becomesinsufficient. When less than 3% by weight, improvement in fusion of theparticles tends to be lowered.

In the second impregnation polymerization, a conjugated diene monomermay be added to an extent not affecting an effect of enhancement offusion. Examples of the conjugated diene monomer used are the conjugateddiene monomers exemplified in the first impregnation polymerization. Theconjugated diene monomer used may be the same as or different from thatin the monomer mixture. Also other monomer may be added to an extent notaffecting an intended break resistance, fusion between the particles andcushioning property.

The polymerization initiator to be used may be the same as that used inthe first impregnation polymerization. The amount of the polymerizationinitiator is usually from 0.05 to 5% by weight, preferably from 0.5 to2% by weight on the basis of the styrene monomer to be used in thesecond impregnation polymerization, but may be decreased when thereremains the polymerization initiator which was not consumed in the firstimpregnation polymerization.

The rubber polymer of rubber polymer particles (b) contained in theso-obtained modified polystyrene resin particles (c) has a basestructure of random copolymer comprising the styrene monomer andconjugated diene monomer because the both of styrene monomer andconjugated diene monomer are radically polymerized in a state of beingcontained uniformly in the polystyrene resin particle (a). The rubberpolymer has such a structure that a part of the random copolymer isgrafted on a polystyrene chain of the polystyrene resin particle (a) ora styrene polymer derived from the styrene monomer impregnated in thesecond impregnation polymerization is graft-polymerized with the polymerobtained in the first impregnation polymerization and further there iscrosslinking therebetween. Thus the modified polystyrene resin particles(c) have a structure having no blocked portion consisting of theconjugated diene monomer unit. Namely the rubber polymer particles (b)contained in the modified polystyrene resin particles (c) of the presentinvention has a structure entirely different from that of rubber polymerparticles (having a long chain consisting of a butadiene unit) ofconventional high impact polystyrene resin (HIPS) prepared bypolymerizing a styrene monomer solely in a state of a styrene-butadieneblock copolymer or a straight chain polybutadiene being dissolved in thestyrene monomer. Also from this point of view, the modified polystyreneresin particles (c) of the present invention can be discriminated fromconventional high impact polystyrene resin (for example, JP-B-47-18428,JP-A-7-90105, JP-A-5-116227, JP-A-8-188669, JP-A-8-245822, U.S. Pat. No.4,409,338, U.S. Pat. No. 5,880,166, U.S. Pat. No. 5,661,191, etc.).

Further since the modified polystyrene resin particles of the presentinvention have a graft structure and crosslinking structure as mentionedabove, they have a gel content higher than that of conventional HIPS. Apreferred gel content is 15 to 40% by weight, further 20 to 35% byweight, particularly 23 to 35% by weight. When the gel content is inthat range, a good fusion between the expanded particles is obtained andbreak resistance and cushioning property of a foamed article areexcellent. In JP-A-6-49263 (particle containing rubber polymer particlesdensely in a surface portion thereof), a gel content of the particledescribed therein is in a wide range of 0.1 to 35% by weight, but thegel content of the actually obtained particle is described as small as1.8 to 9.3% by weight and a particle having a gel content of not lessthan 15% by weight is not prepared.

Also as mentioned above, since conventional high impact polystyrene(HIPS) is prepared by polymerizing a styrene monomer after dissolving arubber polymer in the styrene monomer, polystyrene resin fine particlesare contained substantially in rubber polymer particles being present inHIPS (commonly called occlusion polystyrene (occlusion PS) which can berecognized through a transmission electron microscope by dyeing withosmium oxide. Refer to FIG. 7.). However in the preparation process ofthe present invention for forming the rubber polymer particles (b) byimpregnating the polystyrene resin particles (a) with the monomermixture, so-called occlusion polystyrene is not recognized in the rubberpolymer particles (b) in the obtained modified polystyrene resinparticles (c) even by viewing through a transmission electron microscopeat a magnification of ×40000 by dyeing with osmium oxide (cf. FIG. 1).Namely the rubber polymer particles (b) are featured by consistingessentially of a copolymer of the styrene monomer and the conjugateddiene monomer. From this point of view, the modified polystyrene resinparticles of the present invention can be discriminated from theabove-mentioned conventional HIPS. As mentioned above, the copolymer ofstyrene monomer and conjugated diene monomer may contain other monomeras a non-essential third component in a range not affecting propertiesof the rubber polymer particles (b) as a rubber. When the styrenemonomer is contained in the monomer mixture in a considerably largeamount, there is a case where a copolymer being rich in the styrenemonomer unit is present together, but such a copolymer is not anocclusion polystyrene.

Further since the preparation process of the present invention differsfrom that of conventional high impact polystyrene resin (HIPS) asmentioned above, in the rubber polymer particles (b) contained in themodified polystyrene resin particles (c), there exist many rubberpolymer particles having a shape which cannot be said to be spherical.Particularly there is a feature that relatively large rubber polymerparticles have a corner or are bent (cf. FIG. 1). Also from this pointof view, the resin particles of the present invention can bediscriminated from conventional HIPS and particles prepared byimpregnation polymerization and disclosed in JP-A-6-49263 or WO98/29485.

The foamable modified polystyrene resin particles (d) of the presentinvention are those in which the blowing agent (e) is contained in themodified polystyrene resin particles (c). The blowing agent (e) may beintroduced into the modified polystyrene resin particles (c), may beintroduced together with the monomer mixture during the firstimpregnation polymerization or may be impregnated together with thestyrene monomer during the second impregnation polymerization. When thesecond impregnation polymerization is carried out, it is preferable thatthe impregnation of the blowing agent is done after the secondimpregnation polymerization from the points that a pressure inside areactor is not required to be made high and a dispersion stability isgood.

The blowing agent (e) is preferably of volatile type. For example, thereare one or two or more of butane, pentane, and the like. Among them,preferred is a sole use of butane and pentane or a mixture thereof. Alsoa small amount of cyclohexane, cyclopentane, hexane, and the like may beused together. Particularly pentane is preferred (including a sole useof normal pentane and isopentane or a mixture thereof).

A content of the blowing agent in the foamable modified polystyreneresin particles (d) may be the same as in the case of un-modifiedpolystyrene resin particles, and may be selected in a range of 3 to 15parts by weight, usually 5 to 10 parts by weight on the basis of 100parts by weight of resin particles in consideration of a desiredexpansion ratio.

Further a solvent may be used to improve an expansion rate. Suitablesolvents are, for example, cyclohexane and aromatic hydrocarbons such astoluene, xylene and ethylbenzene.

The present invention further relates to the method of producing afoamed article by expanding the foamable modified polystyrene resinparticles (d) of the present invention and to the foamed article. Asmentioned above, the foamed article of the present invention encompassespre-expanded particles obtained by expanding the foamable modifiedpolystyrene resin particles and also an expansion-molded articleobtained by molding the pre-expanded particles in mold.

First, the pre-expanded particles are explained below. The pre-expandedparticles are obtained by impregnating modified or unmodifiedpolystyrene resin particles with the blowing agent and then heating, forexample, with steam or the like for the expansion. The pre-expansionmethod is a well known technique and can be used for the presentinvention as it is.

The pre-expanded particles of the present invention have breakresistance and can be used as they are for a loose fill type cushioningmaterial. They are particularly useful as a material for a foamedarticle obtained by in-mold molding which is explained next.

The in-mold molding is a well known method for producing a foamedarticle by putting the pre-expanded particles in the mold and heatingfor fusion between the pre-expanded particles. In the present invention,usual pre-expanding conditions may be properly selected.

The thus produced foamed article has sufficient fusion between thepre-expanded particles, and high break resistance is exhibited.

Further the inventors of the present invention have found that when afoamed article is produced by using the foamable modified polystyreneresin particles (d) obtained from the modified polystyrene resinparticles (c) containing the blowing agent (e) and comprising thepolystyrene resin and rubber polymer particles (b) dispersed in acontinuous phase of the resin, particularly a balance of breakresistance and cushioning property is excellent when a 50% failureheight according to falling weight method and a cushioning coefficientof the foamed article satisfy specific equations.

Namely provided that a cushioning coefficient of the modifiedpolystyrene resin foamed article having a density of 0.02 g/cm³ andobtained from the foamable modified polystyrene resin particles (d) isrepresented by A and a 50% failure height according to falling weightmethod of the modified polystyrene resin foamed article is representedby B and that a cushioning coefficient of un-modified polystyrene resinfoamed article having a density of 0.02 g/cm³ and containing no rubberpolymer particles is represented by C and a 50% failure height accordingto falling weight method of the unmodified polystyrene resin foamedarticle is represented by D, particularly useful as a cushioningmaterial is a foamed article having a relation that a cushioningcoefficient ratio A/C is 1.00 to 1.08 and an impact strength ratio B/Dis 1.35 to 2.35. Preferably A/C is 1.00 to 1.06 and B/D is 1.45 to 2.35,more preferably A/C is 1.00 to 1.05 and B/D is 1.45 to 2.35. When thecushioning coefficient ratio (A/C) is not less than 1.08 (represented bya foamed article of conventional HIPS), an energy absorbing ability of afoamed article as a cushioning material is low and cushioningperformance is inferior. When the impact strength ratio (B/D) is notmore than 1.35 (represented by unmodified polystyrene resin foamedarticle), break resistance tends to become insufficient.

Also the inventors of the present invention have found that when themodified polystyrene resin foamed article is obtained by expanding thefoamable modified polystyrene resin particles (d) in which the modifiedpolystyrene resin particles (c) contains the blowing agent (e) and therubber polymer particles (b) are dispersed uniformly throughout acontinuous phase of the polystyrene resin or in a state of a density ofthe particles (b) being higher in the center portion of the resinparticle than in the surface layer portion thereof, the rubber polymerparticles being present in a cell membrane of the obtained foamedarticle are flattened and particularly when the rubber polymer particlesare flattened in an average flatness range of 1.1 to 9, excellent breakresistance and cushioning property are exhibited. A preferable averageflatness is from 1.5 to 8. If the rubber polymer particles are notflattened substantially (average flatness is less than 1.1), breakresistance is insufficient when the resin particles are formed into ahighly expanded foamed article having a density of not more than 0.02g/cm³. If the average flatness is increased, there is a tendency that ashrinkage of pre-expanded particles increases, a cushioning coefficientincreases and cushioning property becomes inferior.

FIG. 5 shows the rubber polymer particles in the cell membrane when thefoamable modified polystyrene resin particles corresponding to Example 2are formed into a foamed article having a density of 0.02 g/cm³. FIG. 6shows the rubber polymer particles in the cell membrane when thefoamable modified polystyrene resin particles of Comparative Example 3corresponding to foamable resin particles described in WO98/29485 areformed into a foamed article having a density of 0.02 g/cm³. As it isclear from a comparison between FIGS. 5 and 6, it can be seen that inthe present invention the rubber polymer particles are adequatelyflattened in the cell membrane and differ in their properties from thoseof the foamable resin particles described in WO98/29485 in which therubber polymer particles are not deformed.

Further the inventors of the present invention have found that withrespect to the foamable modified polystyrene resin particles in whichthe modified polystyrene resin particles contains the blowing agent andthe rubber polymer particles are dispersed uniformly throughout acontinuous phase of the polystyrene resin or in a state of a density ofthe particles being higher in the center portion of the resin particlethan in the surface layer portion thereof, when the rubber polymerparticles being present in the center portion of the foamable modifiedpolystyrene resin particles have a circle equivalent diameter of averagearea of 0.01 to 0.20 μm, a balance of break resistance and cushioningproperty of a foamed article obtained by expanding is excellent. Whenthe circle equivalent diameter of average area is not more than 0.01 μm,an impact strength by a falling weight method tends to be decreased, andwhen not less than 0.20 μm, there is a tendency that a cushioningcoefficient is increased to make cushioning property inferior. Apreferable circle equivalent diameter of average area is from 0.02 to0.18 μm, more preferably 0.04 to 0.16 μm, particularly preferably 0.05to 0.14 μm.

In the present invention, the rubber polymer particles which are notspherical substantially are contained among the rubber polymer particles(b) in the modified polystyrene resin particles (c). This is one of thefeatures of the present invention. In that case, there is a case whereit is difficult to obtain an average particle size of the rubber polymerparticles, and therefore the particle size is represented by the circleequivalent diameter of average area obtained from an area of the rubberpolymer particles as mentioned above.

A difference between the foamable polystyrene resin particles (d) of thepresent invention and some conventional foamable polystyrene resinparticles is as mentioned above. Then a difference from another knownfoamable polystyrene resin particles and foamed article is explainedbriefly in other aspects.

A foamed article obtained from usual high impact polystyrene resin(HIPS) has an impact strength ratio by a falling weight method in a widerange of from 0.6 to 2.0. However from a production condition that astyrene monomer is solely polymerized in the presence of a rubberpolymer (in a dissolved state), a molecular weight of a polystyreneportion is hard to be increased and a re-pelletizing step for forminginto particles is required and a production cost is increased. Also acushioning coefficient ratio of a foamed article of the high impactpolystyrene resin (HIPS) becomes not less than 1.08, and therefore it ishard to say that cushioning property is so excellent. It is understoodin general that unless the rubber polymer particles are relativelylarge, for example, unless a particle size thereof is 1 to 2 μm or more,good properties are not exhibited.

JP-A-8-188669 discloses foamable rubber-modified styrene resin particlesobtained from rubber-modified styrene resin particles which comprise 8to 15% by weight of butadiene rubber particles having a 1,4-cisstructure in a proportion of not less than 70% by weight and an averageparticle size of 1.5 to 3.0 μm and a styrene resin (Z average molecularweight: not less than 350,000) and have a mineral oil content of notmore than 3% by weight and a blowing agent content of from 1 to 20% byweight. In order to increase a proportion of 1,4-cis structure to 70% byweight or more, it is necessary to employ a specific polymerizationmethod specially regulated such as ion polymerization. On the otherhand, in the present invention, the styrene monomer and conjugated dienemonomer are subjected to radical polymerization in an entirelyunconditioned state to give a random copolymer. In that case aproportion of 1,4-cis structure is very low. In that point, the rubberpolymer particles of the present invention completely differ from thosedescribed in JP-A-8-188669. Further a particle size of the rubberpolymer particles of JP-A-8-188669 is the same as that of usual HIPSresin particles, and also from this point of view, the invention ofJP-A-8-188669 differs from the present invention in which the particlesize of the rubber polymer particles is relatively small.

JP-A-7-11043 describes that a foamed article of high impact polystyreneresin (HIPS) in which rubber polymer particles are flattened in a rangeof L/D of 10 to 70 has a high break resistance. However the resin isHIPS and a preparation process thereof differs from that of the presentinvention. Further the rubber polymer particles have a structure likesalami or a core-shell structure and the structure and properties areentirely different from those of the present invention and therefore apreferred average flatness is in a completely different range.

Also JP-A-56-67344 describes a polymer composition obtained by anentirely different preparation process by using a resin which is neitherusual HIPS resin nor the resin of the present invention prepared byimpregnation polymerization. Namely high impact resin particles areprepared by dissolving a straight chain styrene/butadiene blockcopolymer and a straight chain polybutadiene in toluene, removingtoluene by solvent casting method, and then crosslinking polybutadieneat 140° C. In that invention, since the steps of dissolving in thesolvent and then removing the solvent by the solvent casting method areemployed, the rubber portion thereof has a micro phase separationstructure of styrene component and conjugated diene polymer component,and a particle size of the rubber portion is smaller as compared withthat of HIPS resin though the both resins are equal from a point thatthe rubber portions thereof are block copolymers. However thepreparation steps are not practical because a cost is too high. Furtherthe particles of the rubber portion contain a blocked polybutadiene in alarge amount and differ in a structure, properties and characteristicsfrom the rubber polymer particles of the present invention whichbasically comprise a random copolymer of a styrene monomer andconjugated diene monomer.

The features and preferred embodiments of the present invention anddifferences from known techniques are as explained above. Then thepresent invention is explained below by means of examples, but is notlimited to them.

EXAMPLES 1 TO 5

A 5-liter reactor with a stirrer was charged with an aqueous suspensioncomprising 110 parts (part by weight, hereinafter the same) of water,0.013 part of sodium a-olefin sulfonate, 0.44 part of calcium phosphateand polystyrene particles having an average particle size of 1 mm in anamount shown in Table 1, and then was sealed. With stirring, the mixturewas heated to 80° C., and impregnated with a monomer mixture in anamount shown in Table 1. As the initiator, 0.066 part of lauroylperoxide was added, followed by the first impregnation polymerizationfor six hours.

Then the reactor was heated to 90° C. and 10.1 parts of styrene and0.049 part of benzoyl peroxide were added additionally over one hour,followed by heating to 115° C. over one hour and conducting the secondimpregnation polymerization for two hours.

Subsequently 6.6 parts of pressurized pentane(normal-pentane/iso-pentane=40/60) was introduced as a blowing agent andimpregnation of pentane was carried out at 115° C. for two hours. Themixture was then cooled to 30° C. and taken out of the reactor, followedby dehydrating and drying to give foamable modified polystyreneparticles of the present invention.

The obtained foamable resin particles were pre-expanded by about 50times under the above-mentioned conditions to give pre-expandedparticles having a density of 0.02 g/cm³. The pre-expanded particleswere allowed to stand for 24 hours, followed by in-mold molding underthe above-mentioned conditions to give a foamed article.

EXAMPLE 6

A 5-liter reactor with a stirrer was charged with an aqueous suspensioncomprising 110 parts of water, 0.013 part of sodium a-olefin sulfonate,0.44 part of calcium phosphate and polystyrene particles having anaverage particle size of 1 mm in an amount shown in Table 1, and thenwas sealed. With stirring, the mixture was heated to 80° C., andimpregnated with a monomer mixture in an amount shown in Table 1. As theinitiator, 0.066 part of lauroyl peroxide was added, followed by thefirst impregnation polymerization for six hours. Then the reactor washeated to 115° C., followed by post-polymerization for two hours.

Subsequently 6.6 parts of pressurized pentane(normal-pentane/iso-pentane=40/60) was introduced as a blowing agent andimpregnation of pentane was carried out at 115° C. for two hours. Themixture was then cooled to 30° C. and taken out of the reactor, followedby dehydrating and drying to give foamable modified polystyreneparticles of the present invention.

The obtained foamable resin particles were pre-expanded by about 50times in the same manner as in Example 1 to give pre-expanded particleshaving a density of 0.02 g/cm³. The pre-expanded particles weresubjected to in-mold molding to give a foamed article.

COMPARATIVE EXAMPLE 1

Foamable polystyrene particles were obtained in the same manner as inExample 1 except that a blowing agent (normal-pentane/iso-pentane=40/60)was impregnated without modifying (impregnation polymerization) rawpolystyrene particles. The obtained foamable resin particles weresubjected to pre-expanding and in-mold molding in the same manner as inExample 1 to give a foamed article.

COMPARATIVE EXAMPLE 2

A 5-liter reactor with a stirrer was charged with an aqueous suspensioncomprising 96 parts of water, 0.0096 part of sodium α-olefin sulfonate,0.32 part of calcium phosphate and 80 parts of polystyrene particleshaving an average particle size of 1 mm, and then was sealed.

Separately 8 parts of a styrene monomer and 12 parts of butadiene weremixed under pressure to give a liquid monomer mixture. To the monomermixture were added 0.1 part of benzoyl peroxide and 0.025 part oft-butyl peroxybenzoate as a polymerization initiator to give a mixture.

The obtained mixture was added in the reactor and dispersed togetherwith polystyrene particles in an aqueous medium. The dispersion wasstirred at 60° C. for two hours and the mixture was absorbed in thepolystyrene particles. The dispersion was then heated to 90° C. and heldfor four hours and further heated to 125° C. and held for two hours tocopolymerize styrene with butadiene. The dispersion was then cooled to100° C. and thereto was added 10 parts of pressurized pentane as ablowing agent, followed by holding for six hours to impregnate theparticles with the blowing agent. The dispersion was cooled to 30° C.,taken out of the reactor, and then dehydrated and dried to give foamablepolystyrene particles.

This Comparative Example corresponds to the invention disclosed inJP-A-6-49263.

COMPARATIVE EXAMPLE 3

Foamable polystyrene particles were obtained in the same manner as inExample 1 except that an amount of a monomer mixture was changed to thatshown in Table 2. The obtained foamable resin particles were subjectedto pre-expanding and in-mold molding in the same manner as in Example 1to give a foamed article.

This Comparative Example corresponds to the invention disclosed inWO98/29485.

COMPARATIVE EXAMPLE 4

A high impact polystyrene resin (HIPS, rubber content: 8%, molecularweight of a polystyrene portion: 230000) was extruded and cut to givesmall particles of about 1 mg. Those resin particles were impregnatedwith a blowing agent in the same manner as in Example 1 to give foamablehigh impact polystyrene resin particles which were then subjected topre-expanding and in-mold molding in the same manner as in Example 1 togive a foamed article.

COMPARATIVE EXAMPLE 5

Foamable polystyrene particles were obtained in the same manner as inExample 1 except that an amount of a monomer mixture was changed to thatshown in Table 2. The obtained foamable resin particles were subjectedto pre-expanding and in-mold molding in the same manner as in Example 1to give a foamed article.

Comparative Example 5 corresponds to the invention disclosed inWO98/29485.

The results of evaluation of the foamable modified polystyrene resinsand foamed articles in Examples 1 to 6 and Comparative Examples 1 to 5are shown in Table 1 (Examples) and Table 2 (Comparative Examples). InTable 1, Bd and St represent butadiene and styrene, respectively.

TABLE 1 Example 1 2 3 4 5 6 Preparation of foamable PS particles (d)Amount of raw PS particles (a) 79 75 73 68 83 75 (part by weight) Firstimpregnation polymerization Monomer mixture (part by weight) 21 25 27 3217 25 Styrene (% by weight) 57 64 67 72 65 64 Butadiene (% by weight) 4336 33 28 35 36 Second impregnation polymerization Styrene (part byweight) 10.1 10.1 10.1 10.1 10.1 — Physical properties of foamable PSparticles (d) Shape of rubber polymer particles Non- Non- Non- Non- Non-Non- spherical spherical spherical spherical spherical sphericalDistribution of rubber polymer Uniform Uniform Uniform Uniform UniformUniform particles (b) Existence of occlusion PS None None None None NoneNone Area ratio of rubber portion 0.75 0.66 0.64 0.63 0.72 0.90 Circleequivalent diameter of average 0.08 0.09 0.09 0.11 0.07 0.08 area (μm)Gel content (% by weight) 25.3 30.2 32.2 33.4 22.0 26.5 Physicalproperties of foamed article Impact strength according to the 17 23 2018.5 18 22 falling weight method (cm) Impact strength ratio 1.36 1.841.60 1.48 1.44 1.76 Cushioning coefficient 2.93 2.92 2.91 2.90 2.90 2.91Cushioning coefficient ratio 1.05 1.04 1.04 1.04 1.04 1.04 Averageflatness 3.2 3.6 4.0 4.8 3.6 3.6

TABLE 2 Comparative Example 1 2 3 4 5 Preparation of foamable PSparticles (d) Amount of raw PS particles (a) 100 80 90 — 85 (part byweight) First impregnation polymerization Monomer mixture (part byweight) — 20 10 — 15 Styrene (% by weight) — 40 0 — 33 Butadiene (% byweight) — 60 100 — 67 Second impregnation polymerization Styrene (partby weight) — 10 — 10.1 Physical properties of foamable PS particles (d)Shape of rubber polymer particles — Non- Spherical Spherical Non-spherical spherical Distribution of rubber polymer Uniform High UniformUniform Uniform particles (b) density around surface Existence ofocclusion PS — None None Found None Area ratio of rubber portion — 1.170.87 — 0.97 Circle equivalent diameter of average — 0.01 0.01 0.2 0.1area (μm) Gel content (% by weight) — 20 15 15 18 Physical properties offoamed article Impact strength according to the 12.5 15 14 20 15 fallingweight method (cm) Impact strength ratio 1.0 1.2 1.12 1.6 1.2 Cushioningcoefficient 2.80 2.95 2.98 3.04 2.96 Cushioning coefficient ratio 1.001.05 1.06 1.09 1.06 Average flatness — — 1 20 1.0

As mentioned above, the foamable modified polystyrene resin particlesand foamed article of the present invention can provide a foamed articleexcellent in break resistance as shown in Table 1. In ComparativeExamples shown in Table 2, particularly in Comparative Examples 3 and 5,in which foamable modified polystyrene resin particles (WO98/29485)containing rubber polymer particles which are not deformed substantiallyafter expanding are used, impact resistance and break resistance areenhanced.

Industrial Applicability

According to the present invention, there can be provided a modifiedpolystyrene foamed article having excellent break resistance and highfusion rate and foamable modified polystyrene resin particles which areraw materials for producing the foamed article. Such a modifiedpolystyrene resin foamed article can be produced at relatively low costas compared with an expensive high impact polystyrene resin foamedarticle.

The foamed article of the present invention can be used for applicationswhere high impact polystyrene resin foamed articles have been used, andis useful as a shock absorbing material particularly for preciseelectric appliances, for example, OA apparatuses such as CRT and printerand AV apparatuses such as audio system and video tape recorder.

What is claimed is:
 1. Foamable modified polystyrene resin particleswhich are foamable modified polystyrene resin particles (d) comprisingmodified polystyrene resin particles (c) containing a blowing agent (e);said modified polystyrene resin particles (c) comprising a polystyreneresin and rubber polymer particles (b) being dispersed in the resin,wherein the rubber polymer particles (b) are dispersed uniformlythroughout a continuous phase of the polystyrene resin or in a state ofa density of the particles (b) being higher in a center portion of theresin particle than in a surface layer portion thereof, the rubberpolymer particles (b) consist essentially of a copolymer of a styrenemonomer and a conjugated diene monomer and the rubber polymer particles(b) are flattened in a cell membrane of the modified polystyrene resinfoamed article obtained by expanding the foamable modified polystyreneresin particles and an average flatness of the rubber polymer particles(b) flattened in the cell membrane of the modified polystyrene resinfoamed article obtained by expanding the foamable modified polystyreneresin particles (d) is within a range of 1.1 to 9, and the modifiedpolystyrene resin particles (c) are modified polystyrene resin particles(c1) obtained by impregnating a monomer mixture consisting essentiallyof a styrene monomer and a conjugated diene monomer into polystyreneresin particles (a) and carrying out copolymerization, said monomermixture containing the styrene monomer in an amount of more than 55% byweight and not more than 90% by weight and the conjugated diene monomerin an amount of not less than 10% by weight and less than 45% by weight.2. The foamable modified polystyrene resin particles of claim 1, whereinsaid modified polystyrene resin particles (c) are modified polystyreneresin particles (c2) obtained by copolymerizing the monomer mixtureconsisting essentially of the styrene monomer and the conjugated dienemonomer and containing the styrene monomer in an amount of more than 55%by weight and not more than 90% by weight and than the conjugated dienemonomer in an amount of not less than 10% by weight and less than 45% byweight with the monomer mixture being impregnated in the polystyreneresin particles (a) to give the modified polystyrene resin particles(c1) and further subjecting the resin particles (c1) to impregnationpolymerization of a styrene monomer.
 3. The foamable modifiedpolystyrene resin particles of claim 1, wherein said modifiedpolystyrene resin particles (c1) are prepared by impregnating 40 to 10parts by weight of the monomer mixture into 60 to 90 parts by weight ofpolystyrene polymer particles (a) and carrying out copolymerization. 4.The foamable modified polystyrene resin particles of claim 2, whereinsaid modified polystyrene resin particles (c1) are prepared byimpregnating 40 to 10 parts by weight of the monomer mixture into 60 to90 parts by weight of polystyrene polymer particles (a) and carrying outcopolymerization.
 5. The foamable modified polystyrene resin particlesof claim 1, wherein provided that when the foamable modified polystyreneresin particles (d) are molded into a modified polystyrene resin foamedarticle having a density of 0.02 g/cm³, a cushioning coefficient isrepresented by A and a 50% failure height according to falling weightmethod of the foamed article is represented by B, and that when foamableun-modified polystyrene resin particles (f) prepared by impregnating thepolystyrene resin particles (a) with the blowing agent (e) are moldedinto an un-modified polystyrene resin foamed article having a density of0.02 g/cm³, a cushioning coefficient is represented by C and a 50%failure height according to falling weight method of the foamed articleis represented by D, the modified polystyrene resin foamed article has arelation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.
 6. Thefoamable modified polystyrene resin particles of claim 1, wherein therubber polymer particles (b) being present in the center portion of thefoamable modified polystyrene resin particles (d) have a circleequivalent diameter of average area of 0.01 to 0.20 μm and a gel contentof the modified polystyrene resin particles (c) is from 15 to 40% byweight.
 7. The foamable modified polystyrene resin particles of claim 4,wherein the rubber polymer particles (b) being present in the centerportion of the foamable modified polystyrene resin particles (d) have acircle equivalent diameter of average area of 0.01 to 0.20 μm and a gelcontent of the modified polystyrene resin particles (c) is from 15 to40% by weight.
 8. The foamable modified polystyrene resin particles ofclaim 4, wherein provided that when the foamable modified polystyreneresin particles (d) are molded into a modified polystyrene resin foamedarticle having a density of 0.02 g/cm³, a cushioning coefficient isrepresented by A and a 50% failure height according to falling weightmethod of the foamed article is represented by B, and that when foamableunmodified polystyrene resin particles (t) prepared by impregnatingpolystyrene resin particles (a) with the blowing agent (e) are moldedinto an un-modified polystyrene resin foamed article having a density of0.02 g/cm³, a cushioning coefficient is represented by C and a 50%failure height according to falling weight method of the foamed articleis represented by D, the modified polystyrene resin foamed article has arelation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.
 9. Foamablemodified polystyrene resin particles which are foamable modifiedpolystyrene resin particles (d) comprising modified polystyrene resinparticles (c) containing a blowing agent (e); said modified polystyreneresin particles (c) comprising a polystyrene resin and rubber polymerparticles (b) being dispersed in the resin, wherein the rubber polymerparticles (b): are dispersed uniformly throughout a continuous phase ofthe polystyrene resin or in a state of a density of the particles (b)being higher in a center portion of the resin particle than in a surfacelayer portion thereof, consist essentially of a copolymer of a styrenemonomer and a conjugated diene monomer and are flattened in a cellmembrane of a modified polystyrene resin foamed article obtained byexpanding the foamable modified polystyrene resin particles (d); and anaverage flatness of the rubber polymer particles (b) flattened in a cellmembrane of the modified polystyrene resin foamed article obtained byexpanding the foamable modified polystyrene resin particles (d) andhaving a density of 0.02 /cm³ is within a range of 1.1 to
 9. 10. Thefoamable modified polystyrene resin particles of claim 9, whereinprovided that when the foamable modified polystyrene resin particles (d)are molded into a modified polystyrene resin foamed article having adensity of 0.02 g/cm³, a cushioning coefficient is represented by A anda 50% failure height according to falling weight method of the foamedarticle is represented by B, and that when foamable un-modifiedpolystyrene resin particles (f) prepared by impregnating polystyreneresin particles (a) with the blowing agent (e) are molded into anun-modified polystyrene resin foamed article having a density of 0.02g/cm³, a cushioning coefficient is represented by C and a 50% failureheight according to falling weight method of the foamed article isrepresented by D, the modified polystyrene resin foamed article has arelation that A/C is 1.00 to 1.08 and B/D is 1.35 to 2.35.
 11. Thefoamable modified polystyrene resin particles of claim 9, wherein therubber polymer particles (b) being present in the center portion of thefoamable modified polystyrene resin particles (d) have a circleequivalent diameter of average area of 0.01 to 0.20 μm and a gel contentin the modified polystyrene resin particles (c) is from 15 to 40% byweight.
 12. The foamable modified polystyrene resin particles of claimwherein the rubber polymer particles (b) being present in the centerportion of the foamable modified polystyrene resin particles (d) have acircle equivalent diameter of average area of 0.01 to 0.20 μm, a gelcontent in the modified polystyrene resin particles (c) is from 15 to40% by weight.
 13. The modified polystyrene resin foamed articleobtained by expanding the foamable modified polystyrene resin particlesof claim
 4. 14. The modified polystyrene resin foamed article obtainedby expanding the foamable modified polystyrene resin particles of claim8.
 15. The modified polystyrene resin foamed article obtained byexpanding the foamable modified polystyrene resin particles of claim 10.16. The modified polystyrene resin foamed article obtained by expandingthe foamable modified polystyrene resin particles of claim
 12. 17. Aprocess for preparing foamable modified polystyrene resin particlescomprising: impregnating, into polystyrene resin particles (a), amonomer mixture consisting essentially of a styrene monomer and aconjugated diene monomer and comprising the styrene monomer in an amountof more than 55% by weight and not more than 90% by weight and than theconjugated diene monomer in an amount of not less than 10% by weight andless than 45% by weight and carrying out copolymerization to givemodified polystyrene resin particles (c1), impregnation-polymerizing themodified polystyrene resin particles (c1) with a styrene monomer to givemodified polystyrene resin particles (c2), and impregnating the modifiedpolystyrene resin particles (c2) with a blowing agent (e).
 18. Theprocess for preparing foamable modified polystyrene resin particles ofclaim 17, wherein in the impregnation polymerization for preparing themodified polystyrene resin particles (c1), 40 to 10 parts by weight ofthe monomer mixture is impregnated into 60 to 90 parts by weight of thepolystyrene polymer particles (a).
 19. The foamable modified polystyreneresin particles of claim 1, wherein the average flatness of the rubberpolymer particles (b) flattened in a cell membrane of the modifiedpolystyrene resin foamed article obtained by expanding the foamablemodified polystyrene resin particles (d) and having a density of 0.02g/cm³ is within a range of 1.5 to
 8. 20. The foamable modifiedpolystyrene resin particles of claim 9, wherein the average flatness ofthe rubber polymer particles (b) flattened in a cell membrane of themodified polystyrene resin foamed article obtained by expanding thefoamable modified polystyrene resin particles (d) and having a densityof 0.02 g/cm³ is within a range of 1.5 to 8.